This application claims priority of European Patent Application No. 00303807.2, which was filed on May 5, 2000.
The present invention relates to radio communication systems, in particular but not exclusively, to mobile radio communication systems.
Mobile radio communication systems involve an array of cells each served by a respective base station. Each base station communicates with various mobile stations located within its cell. Radio transmissions between a base station and a selected mobile station are subject to a variety of impairments. In time division multiple access (TDMA) cellular systems like GSM (global systems for mobile communications) or EDGE (enhanced data for GSM evolution), the received signal is distorted by the time varying multipath propagation channel which causes intersymbol interference. In addition, transmissions are impaired by thermal noise that results from both environmental noise and the noise generated by the circuitry of the receiver. Mobile station and base station radio receivers operate on one or more noisy and distorted waveforms as obtained by one or more receiver antennas. In order to extract the original transmitted data from the received waveforms, TDMA receivers employ equalisation systems which try to compensate for signal distortion and additive disturbances. Such equalisers operate on the basis that each signal burst includes a unique set of training symbols. Traditionally, TDMA cellular receivers employ trellis based receivers for one or more received signals. By using known training symbols transmitted within each burst, the trellis processor of the equalisation system is able to detect the transmitted payload from the noisy and distorted received signals. Conventionally, trellis equalisers operate on a maximum likelihood (ML) (see G D Forney Jr, xe2x80x9cMaximum likelihood sequence estimation of digital sequences in the presence of intersymbol interferencexe2x80x9d, IEEE Trans Inform Theory, vol IT-18, no 3, pp 363-378 May 1972) or a maximum a posterion probability (MAP) (see L R Bahl, J Cocke, F
Jelinek and J Raviv, xe2x80x9cOptimal decoding of linear codes for maximising symbol error ratexe2x80x9d, IEEE Trans Inform Theory, Vol IT-20, pp 284-287, March 1974) criteria and operate optimally based on the assumption that the noise has a Gaussian distribution.
An additional problem arises in the presence of co-channel interference (CCI). To increase spectral efficiency, cellular radio systems usually employ frequency reuse. While the base stations of immediately adjacent cells operate at different frequencies, the number of frequencies available is limited and so some cells further away operate at the same frequency. As the cells become smaller they become closer together and so interference between cells increases, ie a base or mobile station in one cell receives not only a signal of frequency f from the mobile or base station with which a link has been established, but also an interference signal of frequency f from a mobile or base station in another cell operating at that same frequency. Because such interference signals are not Gaussian, the trellis based equalisation system requires a proper spatio-temporal model of the CCI. If, as is so often the case, the disturbance cannot be accurately modelled or the estimated, the trellis based equaliser becomes sub-optimum and experiences a significant performance degradation. Even when a suitable model for the CCI can be developed, the resulting receiver is often too complex to be efficiently implemented. In an interference limited scenario, spatio-temporal filtering techniques (see A M Kuzminskiy, C Luschi and P Strauch, xe2x80x9cComparison of linear and MISE spatio-temporal interference rejection combing with an antenna array in a GSM systemxe2x80x9d in Proc VTC 2000, Tokyo, Japan, May 2000) provide superior performance especially for multiple antenna receivers. However, such techniques perform poorly in noise limited scenarios.
Receivers for implementing a space-time filtering stage followed by a trellis processor have been proposed (see A J Paulraj and C B Papadias xe2x80x9cSpace-time processing for wireless communicationxe2x80x9d IEEE Signal Proc Mag, pp 40-84, November 1997). However, particularly in a software implementation of such an equaliser, this strategy will result in a significant increase in implementation complexity.
It is an object of the present invention to provide an improved radio communication system which is able to deal with both noise and interference disturbances in a more efficient and less costly manner.
According to the present invention there is provided a cellular radio communication system comprising a plurality of cells each having a base station serving a plurality of mobile stations, at least one of said stations having a receiver for receiving a data signal and any accompanying noise propagated from the station with which a link has been established, and an interference signal propagated from a station in another cell, said at least one station having a trellis based equalisation system operative in a sense to detect transmitted data symbols from the accompanying disturbances in the distorted received signal, a spatio-temporal filter operative to process the received signal in a sense to minimise the signal distortion and the associated disturbances, and a switch responsive to predetermined criteria related to the value of the power of the noise signal relative to the value of the power of interference signal in the received signal to switch the received signal from the trellis based equalisation system to the spatio-temporal filter and vice versa.